Summary of Work: A new program has been initiated to study the molecular basis for the decline in T cell function that accompanies aging. Based on the belief that a better understanding of the biochemistry involved in normal T cell activation and function is required in order to identify the precise changes that occur with age, studies have focused on the early membrane-proximal signaling events that occur upon T cell antigen receptor (TCR) activation in standard (non-aged) T cell model systems. An especially interesting TCR-proximal signaling molecule is ZAP-70, which is a protein tyrosine kinase that is expressed only within T cells and NK cells, and is required for T cell activation through the TCR. Of particular interest is the identification of additional signaling molecules that either perpetuate or regulate the activity of ZAP-70. Progress has been made towards this goal by the use of a two-hybrid screening protocol, which has identified four gene products that appear to specifically associate with ZAP-70. The ability of the proteins expressed by these cDNAs to bind ZAP-70 is being evaluated, as is the functional relevance of their association with ZAP-70. T cells isolated from aged animals often show a diminished capacity to cope with oxidative stress. Given observations that certain oxidative stimuli (ultraviolet light and peroxide) are able to recapitulate the signals generated by antigen engagement of the TCR, investigation of the causes of the changes in tolerance towards oxidative stress with age may provide additional insight into the biochemical changes responsible for reduced T cell function with age. By using a T cell line that lacks ZAP-70, we have begun to assess the role played by ZAP-70 in the propagation of stress signals in T cells. Preliminary results show that ZAP-70 is required in order to see activation of MAP kinases in response to oxidative stimuli.